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    SubjectRe: [PATCH 2/2] msync: start async writeout when MS_ASYNC
    On Thu, 31 May 2012 22:43:55 +0200
    Paolo Bonzini <> wrote:

    > msync.c says that applications had better use fsync() or fadvise(FADV_DONTNEED)
    > instead of MS_ASYNC. Both advices are really bad:
    > * fsync() can be a replacement for MS_SYNC, not for MS_ASYNC;
    > * fadvise(FADV_DONTNEED) invalidates the pages completely, which will make
    > later accesses expensive.
    > Having the possibility to schedule a writeback immediately is an advantage
    > for the applications. They can do the same thing that fadvise does,
    > but without the invalidation part. The implementation is also similar
    > to fadvise, but with tag-and-write enabled.
    > One example is if you are implementing a persistent dirty bitmap.
    > Whenever you set bits to 1 you need to synchronize it with MS_SYNC, so
    > that dirtiness is reported properly after a host crash. If you have set
    > any bits to 0, getting them to disk is not needed for correctness, but
    > it is still desirable to save some work after a host crash. You could
    > simply use MS_SYNC in a separate thread, but MS_ASYNC provides exactly
    > the desired semantics and is easily done in the kernel.
    > If the application does not want to start I/O, it can simply call msync
    > with flags equal to MS_INVALIDATE. This one remains a no-op, as it should
    > be on a reasonable implementation.

    Means that people will find that their msync(MS_ASYNC) call will newly
    start IO. This may well be undesirable for some.

    Also, it hardwires into the kernel behaviour which userspace itself
    could have initiated, with sync_file_range(). ie: reduced flexibility.

    Perhaps we can update the msync.c code comments to direct people to

    One wonders how msync() works with nonlinear mappings. I guess
    "badly". I think this was all discussed when we merged
    remap_file_pages() (what a mistake that was) and we decided "too hard".

     \ /
      Last update: 2012-06-14 00:01    [W:0.022 / U:5.132 seconds]
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